CirrusCI fixes (#1786)

* Fix executable name, that doesn't always exist on windows

* Use the `python3` executable on macOS Cirrus

* Homebrew changed where it puts the python binaries

* Update examples to match new config

test/test_from_sdist.py

@@ -69,7 +69,7 @@ def test_simple(tmp_path):
         assert os.path.exists('{package}/setup.py')
         """,
     )
-    setup_py_assertion_cmd = f'python3 -c "{setup_py_assertion_snippet !s}"'
+    setup_py_assertion_cmd = f'python -c "{setup_py_assertion_snippet !s}"'
 
     # build the wheels from sdist
     actual_wheels = cibuildwheel_from_sdist_run(